This invention relates generally to nondestructive testing, and more particularly to a method for automatically adjusting ultrasonic testing systems to account for temperature variations in ultrasonic testing wedges.
Nondestructive testing devices can be used to inspect test objects to identify and analyze flaws and defects in the objects both during and after an inspection. Nondestructive testing allows an operator to maneuver a probe at or near the surface of the test object in order to perform testing of both the object surface and underlying structure. Nondestructive testing can be particularly useful in some industries, e.g., aerospace, power generation, and oil and gas recovery and refining, where object testing must take place without removal of the object from surrounding structures, and where hidden defects can be located that would otherwise not be identifiable through visual inspection.
One example of nondestructive testing is ultrasonic testing. When conducting ultrasonic testing, an ultrasonic pulse can be emitted from a probe and passed through a test object at the characteristic sound velocity of that particular material. The sound velocity of a given material depends mainly on the modulus of elasticity, temperature and density of the material. Application of an ultrasonic pulse to a test object causes an interaction between the ultrasonic pulse and the test object structure, with sound waves being reflected back to the probe. The corresponding evaluation of the signals received by the probe, namely the amplitude and time of flight of those signals, can allow conclusions to be drawn as to the internal quality of the test object without destroying it.
Generally, an ultrasonic testing system includes a probe for sending and receiving signals to and from a test object, a probe cable connecting the probe to an ultrasonic test unit, and a screen or monitor for viewing test results. The ultrasonic test unit can include power supply components, signal generation, amplification and processing electronics, and device controls used to operate the nondestructive testing device. Some ultrasonic test units can be connected to computers that control system operations, as well as test results processing and display. Electric pulses can be generated by a transmitter and can be fed to the probe where they can be transformed into ultrasonic pulses by ultrasonic transducers. Ultrasonic transducers incorporate piezoelectric ceramics which can be electrically connected to a pulsing-receiving unit in the form of an ultrasonic test unit. Portions of the surfaces of the piezoelectric ceramics can be metal coated, forming electrodes that can be connected to the ultrasonic test unit. During operation, an electrical waveform pulse is applied to the electrodes of the piezoelectric ceramic causing a mechanical change in ceramic dimension and generating an acoustic wave that can be transmitted through a material such as a metal or plastic to which the ultrasonic transducer is coupled. Conversely, when an acoustic wave reflected from the material under inspection contacts the surface of the piezoelectric ceramic, it generates a voltage difference across the electrodes that is detected as a receive signal by the ultrasonic test unit or other signal processing electronics.
The amplitude, timing and transmit sequence of the electrical waveform pulses applied by the pulsing unit can be determined by various control means incorporated into the ultrasonic test unit. The pulse is generally in the frequency range of about 0.5 MHz to about 25 MHz, so it is referred to as an ultrasonic wave from which the equipment derives its name. As the ultrasonic pulses pass through the object, various pulse reflections called echoes occur as the pulse interacts with internal structures within the test object and with the opposite side (backwall) of the test object. The echo signals can be displayed on the screen with echo amplitudes appearing as vertical traces and time of flight or distance as horizontal traces. By tracking the time difference between the transmission of the electrical pulse and the receipt of the electrical signal and measuring the amplitude of the received wave, various characteristics of the material can be determined. Thus, for example, ultrasonic testing can be used to determine material thickness or the presence and size of imperfections within a given test object.
Many ultrasonic transducers are phased arrays comprising single or multiple rows of electrically and acoustically independent or isolated transducer elements. A linear array of independent transducer elements can form what is referred to as a transducer pallet comprising a plurality of independent transducer elements. In these types of transducers, each transducer element may be a layered structure comprising a backing block, flexible printed circuit board (“flex circuit”), piezoelectric ceramic layer, and acoustic matching layer. This layered structure is often referred to as an acoustic stack. The various components of the acoustic stack can be bonded together using an adhesive material (e.g., epoxy) and high pressure in a lamination process. Typically, one or more flex circuits can be used to make electrical connections from the piezoelectric ceramic to the ultrasonic test unit, or to a bundle of coaxial cables that ultimately connect to the ultrasonic test unit or other signal processing electronics.
Ultrasonic testing systems typically employ a variety of probes depending on the test object, test object material composition, and environment in which the testing is being performed. For example, a straight-beam probe transmits and receives sound waves perpendicular to the surface of the object being tested. A straight-beam probe can be particularly useful when testing sheet metals, forgings and castings. In another example, a TR probe containing two elements in which the transmitter and receiver functions are separated from one another electrically and acoustically can be utilized. A TR probe can be particularly useful when inspecting thin test objects and taking wall thickness measurements. In yet another example, an angle-beam probe that transmits and receives sound waves at an angle to the material surface can be utilized. An angle-beam probe can be particularly useful when testing welds, sheet metals, tubes and forgings.
In some applications, e.g., when testing pipe welds, the probe can be mounted on a wedge that provides intermediary physical contact between the probe and the test object. Because the test object is typically of a different temperature than the wedge, the temperature of the wedge often changes as an inspection progresses. This temperature variation in the wedge introduces error into the ultrasonic testing process as the temperature variation of the wedge changes the velocity and attenuation of sound waves traveling through it. This, in turn, can result in transducer sound waves missing the intended point of interest and producing erroneous results. For example, in conducting a pipe weld inspection variations in wedge temperature may result in the ultrasonic pulse missing the known internal weld location and being directed to another location within the pipe.
Ultrasonic signals pass through the wedge and are refracted upon entering the test object. The refracted angle of the ultrasonic signal is dependent on Snell's Law: the sine of the refracted angle is directly proportional to the ratio of the speed of sound in the material used to construct the wedge divided by the speed of sound in the material of the test object. Wedges can be made from any material that has an acoustic velocity different from that of the test object, but are typically manufactured from plastics such as plexi-glass or polystyrene material. The speed of sound in these materials varies widely with changes in temperature, thereby causing significant changes in refracted angles. In turn, changes in refracted angles of only a few degrees can direct the ultrasonic sound beam away from a point of interest, resulting in missed defects and erroneous results.
Compensating for thermal changes in the wedge is currently a manual process requiring calibration of the system based on measured environmental conditions. To calibrate the system, the ultrasonic testing system is removed from the test object and the wedge is brought to the same temperature as that of the test object, typically between −40 degrees C. to 100 degrees C. or higher. Once this has occurred, a calibration object with a known defect is attached and tested, and the sound angle of the probe adjusted until the defect appears at its known location. In order to perform such calibration, the ultrasonic testing system has to be removed from and re-attached to the test object each time the system is calibrated. This time and resource consuming calibration process has to be repeated after taking several measurements on the actual inspection target in order to ensure accurate results throughout the testing process.
Furthermore, the current calibration approach fails to take into account temperature gradients that exist within the wedge. As such the current calibration approach is based on an assumed constant temperature of the wedge and test object, the temperature of each being taken at a given point in time. In reality, the temperatures of both the wedge and test object change over time. In addition, the current approach assumes that the temperature of the wedge is consistent throughout the wedge material, when in reality it varies depending on what point on or within the wedge the temperature is taken. Therefore, despite the attempted calibration, subsequent testing is likely to have some degree of error and unreliability as either the angle or amplitude of the sound beam emitted by the transducers and introduced into the test object could be slightly askew, thereby missing or mischaracterizing defects within an object.
It would be advantageous to provide an apparatus and method for automatically adjusting transducer firing parameters to adjust for temperature gradients within the wedge, thereby reducing and/or eliminating the need for time consuming, resource intensive and unreliable manual calibration procedures.